The present invention pertains to films which are to be heat sealed, especially flexible packaging films.
Packaging films are frequently heat sealed in order that the film is converted to a packaging article, such as a bag, pouch, backseamed casing, etc. Irradiation of the polymer in the film, such as E-beam irradiation, crosslinks the polymer in the film, making it stronger. If the seal layer is irradiated, the crosslinking of the polymer can result in a seal layer which has diminished flow characteristics. However, if it is desired to irradiate other layers of the film, it may also be necessary to irradiate the seal layer. It would be desirable to provide a film which can be made stronger by irradiation, without substantially diminishing the heat seal performance of the seal layer.
The present invention is directed to an irradiated multilayer film having a seal layer containing a blend of a highly branched homogeneous polymer with a semicrystalline polymer. This blend can be irradiated without substantially diminishing the heat seal performance of the seal layer. The highly branched homogeneous polymer can be subjected to irradiation without resulting in a crosslinked structure which diminished the heat seal performance of the seal layer. The highly branched homogeneous polymer also prevents the semicrystalline polymer from crosslinking in a manner which substantially reduces heat seal performance.
As a first aspect, the present invention is directed to an irradiated multilayer film has a heat seal layer containing a blend of a highly branched homogeneous polymer and a semicrystalline polyolefin. The highly branched homogeneous polymer has at least 40 branches per 1000 methylene groups, has a Mw/Mn less than 3.0, and a density of less than 0.89 g/cc. The highly branched homogeneous polymer is present in an amount of from about 5 to 50 weight percent (preferably, 20 to 40 weight percent) based on layer weight. The semicrystalline polyolefin has a density of at least 0.90 g/cc, and is present in an amount of from about 50 to 95 weight percent (preferably, from 60 to 80 weight percent), based on total layer weight. The second layer contains a crosslinked polymer network. Preferably, the heat seal has a seal strength at 75xc2x0 C. of at least 100 grams per centimeter. The seal strength is preferably from 100 to 2000 grams per centimeter, more preferably from 200 to 1000 grams per centimeter. Preferably, the seal has a strength of at least 200 grams per centimeter.
Preferably, the semicrystalline polyolefin has a density of at least 0.905 g/cc, more preferably at least 0.91 g/cc, and can even have a density of at least 0.92 g/cc.
Preferably, the highly branched homogeneous polymer has from 40 to 120 branches per 1000 methylene groups, more preferably, from 40 to 90 branches per 1000 methylene groups, and still more preferably, from 45 to 80 branches per 1000 methylene groups.
Preferably, the semicrystalline polyolefin comprises at least one member selected from the group consisting of very low density polyethylene, linear low density polyethylene, low density polyethylene, high density polyethylene, propylene homopolymer, propylene copolymer, linear homogeneous ethylene/alpha-olefin copolymer, homogeneous ethylene/alpha-olefin copolymer having long chain branching. The semicrystalline polyolefin is different from the highly branched homogeneous polymer in that the semicrystalline polymer, if branched, has less than 60 branches per 1000 methylene groups. Preferably, the highly branched homogeneous polymer is a homopolymer of ethylene.
In one embodiment, the multilayer film comprises an oxygen barrier layer. Preferably, such a multilayer film further comprises a tie layer between the oxygen barrier layer and the seal layer. Preferably, the heat seal layer is an inside layer of the packaging article, and the tie layer between the oxygen barrier layer and the seal layer is a first tie layer, with the multilayer film further comprising a second tie layer between the oxygen barrier layer and an outside layer.
In one embodiment, the film is heat-shrinkable. Preferably, such a film has a total free shrink, at 185xc2x0 F., of at least 20 percent; preferably, from 20 to 120 percent; more preferably, from 20 to 80 percent. In another embodiment, the multilayer film has a total free shrink, at 185xc2x0 F., of less than 10 percent.
Preferably, the film has a thickness of from about 0.3-15 mils; more preferably, from 0.5 to 10 mils; or 1.5 to 6 mils, or 1.5 to 5 mils, or 1.5 to 4 mils, or 2 to 4 mils.
As a second aspect, the present invention is directed to a process for making a packaging article. The process comprises coextruding a multilayer film comprising a first layer which is a heat seal layer, and a second layer containing a thermoplastic polymer. After coextrusion of the first and second layers, the resulting multilayer film is irradiated to crosslink the thermoplastic polymer of the second layer. The heat seal layer is then sealed to itself or another component of the packaging article. The heat seal layer is in accordance with the first aspect of the present invention. Preferably, the heat seal has a seal strength of at least 75 grams per centimeter.